Chunks of shrapnel from unexplained explosions are hurtling through space at 22,000 mph and forcing designers to add tons of shielding to protect the proposed U.S. space station and its crew.

Nearly 7,100 pieces of orbiting debris are being tracked by the U.S. Space Command and more than half came from satellites or spent rockets that "have blown up or broken up for unknown reasons," a NASA expert says.More than 90 satellites to date have shattered unaccountably, but Don Kessler, project scientist for debris studies at the Johnson Space Center in Houston, said he doubts the cause is collisions with meteoroids or tests of secret anti-satellite weapons.

"They are like a time bomb waiting to explode," Kessler said in a telephone interview. "These explosions have occurred everywhere, from one day (after launch) to three years."

Space station planners, "faced with a considerable design problem," are adding 2,000 pounds to the shielding of each of the six modules occupied by astronauts.

"The debris gets most severe at 500-600 miles," Kessler said, "but usually unmanned spacecraft are at that altitude and don't require the same level of safety as a crew." The space shuttle generally operates at altitudes of 150 to 300 miles and so will the space station.

He said officials have long believed that French-made Ariane rockets explode in space but their orbits are difficult to track with ground radar. Two years ago, however, an Ariane was launched into an easy-to-track orbit and, Kessler said, "it blew up after nine months." The fragments of that explosion probably caused by over pressurization of gases number 450.

Americans and Soviets contribute to the space junk, too. Six U.S. Delta rocket second stages exploded, the last in 1981 three years after it was launched. The cause was leakage of fuels that burn when they combine and the problem was fixed in later models.

Such explosions cause an expanding cloud of debris "a problem analogous to cars driving around a parking lot in random directions you can get hit from any direction and the most likely direction will be head-on," Kessler said.

Maj. Alex Mondragon of the U.S. Space Command in Colorado Springs, Colo., said 5,331 pieces of debris and 1,759 "working payloads" were being tracked as of April 19. The objects ranged in size from the Soviet Mir space station to a screwdriver dropped by a spacewalking astronaut.

Space shuttle Challenger returned from a mission in June 1983 with a chip in a cockpit window and chemical analysis showed the culprit was a flake of the paint used on rockets and payloads.

The few objects returned from space all have shown debris damage. The Palapa and Westar satellites that were retrieved by a space shuttle crew were pitted.